As the methods for forming images from an information which is to e visualized, the methods for forming images through electrostatic latent images, such as an electrophotographic method, an electrostatic recording method, an electrostatic printing method, and so forth, have been widely utilized.
In the electrophotographic method, for example, a toner image is formed in such a manner that, after a uniform electrostatic charge is applied to a latent image carrier provided thereon with a light-sensitive layer comprising of a photoconductive material, an electrostatic latent image is formed on the surface of the latent image carrier by image-wise exposing the carrier to light so as to Correspond to an original image, and the electrostatic latent image is developed with a developer. The resulted toner image is transferred onto a recording member such as paper and the like, and the transferred image is then fixed by heating, pressing or the like, so that a copy image can be formed. The latent image carrier subjected to an image-transferring step is electrically neutralized, and the toner remaining untransferred on the latent image carrier is cleaned up to be used for the next copy image formation.
In order to form stably a copy image with high quality and free of fog over many times, it is further required that triboelectric charge of toners is kept stable.
There have been well-known examples of the developers applicable to electrophotographic methods including an electrostatic image developer comprising of toner particles containing polyesters as the binder resins, inorganic fine particles comprising negatively chargeable fine particle silica, and fluoro resin-coated carriers, described in Japanese Patent Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 60-176052/1985; another electrostatic image developer comprising of toner particles containing polyesters as the binder resins, positively chargeable inorganic fine particles comprising of alumina, titanium oxide and nitrogen-containing silica, and so forth, and fluoro resin-coated carriers, described in Japanese Patent O.P.I. Publication No. 62-229158/1987: and so forth.
According to the technique using a combination 0f negatively chargeable fine particle silica and fluoro resin carriers having an intensive negative chargeability, which is disclosed in the above-given Japanese Patent O.P.I. Publication No. 60-176052/1985, the toner particles have slow start in triboelectric charging, and due to toner scattering in a developing unit as a copying operation cycle increases, a charging electrode, a transfer electrode, a separation electrode and so forth are stained, so that poor transferring or forming of image is liable to be caused In addition to the above, when toner scattering increases, a back of leading edge of a transfer member is liable to be stained with toner.
In the meantime, fluoro resins which are used as a coating layer for a fixing roller, such as polytetrafuloroethylene, polyfluoroethyleneproylene and so forth, have an excellent electric insulation property and are easily charged to negative. Based on the above-mentioned fact. Japanese Patent O.P.I. Publication No. 62-229158/1987 discloses a technique using positively chargeable inorganic fine particles, where the toner particles show good start in triboelectric charging and therefore, poor image forming caused by toner scattering may be reduced, but on the contrary, an electrostatic adhesion to a negatively charged fixing roller increases due to high positive charge of toner particles, so that an anti-offset property is worsened and a replacement cycle of a cleaning roller is shortened
On the other hand, the techniques for improving an anti-offset property have been disclosed in, for example, Japanese Patent Examined Publication No. 58-23636/1983, in which a low resistive substance is incorporated a coating layer for a heat roller: Japanese Patent O.P.I. Publication Nos. 57-150869/1982, 59-83181/1984 and 59-111177/1984, in which a conductive substance is incorporated into a primer layer: Japanese Patent O.P.I. Publication No. 61-100777/1986, in which oxobenzoyl polyether and carbon black are incorporated into polytetrafluoroethylene used as a coating layer for a heat roller; and so forth.
According to the above-mentioned techniques, lapping of an offset type paper on a heat roller is improved by conductivity of a coating layer for a heat roller On the other hand, there are defects that in the case a low resistive transfer paper is used or where a transfer paper absorbs moisture under a circumstance of high temperature and humidity and a resistance of the transfer paper is lowered, a transfer current leaks from the heat roller, so that poor transferring is caused and a transfer efficiency is lowered